He's not saying the vaccines are unsafe but that we are, for the first time ever, implementing mass vaccination in the midst of a heated pandemic with a somewhat mismatched vaccine which won't provide sterilizing immunity - with many people getting infected in the period between getting vaccinated and developing immunity - and this will not only drive the emergence of variants which completely evade the vaccine-acquired immunity but the vaccinated will be less able to fight off those variants than if they hadn't been vaccinated since the then ineffective vaccine-acquired immunity, having some affinity for the virus, will take precedence over the immune system's innate response to novel viral attacks.
He also explains the mechanics of pandemics and why the waves of this pandemic are somewhat different than in the past very interestingly, seeming to have an excellent grasp of the situation. If you haven't watched the YouTube video which I linked to, which is a scientific presentation, I think you'll find it at least interesting (and his two YouTube interviews explain his position further). He has excellent credentials, if you look at his LinkedIn page, and says he feels so confident in what he is saying he isn't afraid to put his professional reputation on the line. He's asking scientists to evaluate and comment.